1. The Field of the Invention
The present invention relates to transparent glass ceramics having low thermal expansion, also called “zero expansion”, containing only environmentally acceptable fining agents, or being fined only with environmentally acceptable fining agents, respectively, as well as to the uses of the glass ceramics according to the present invention.
2. The Related Art
Among glass ceramics having low thermal expansion, lithium-aluminium-silicon oxide glass ceramics, which are also known in the art as LAS glass ceramics, are so-called “zero expansion materials”. A well-known type of this sort of glass ceramic material, known as ZERODUR®, is commercially available in three different thermal expansion classes of 0 to 2 and has different coefficients of linear thermal expansion a in a temperature range of 0° to 50° C. as shown in Table I herein below.
TABLE ILinear Thermal Expansion Coefficients of Different ClassesCoefficients of linearExpansion classthermal expansion α00 ± 0.02 · 10−6/K10 ± 0.05 · 10−6/K20 ± 0.10 · 10−6/K
With respect to melting of glass the term “fining” means the removal of gas bubbles from the melt. To achieve the highest quality in terms of removal of foreign gas and bubbles a thorough mixing and degasifying of the molten mixture is necessary. The behavior of gases and bubbles, respectively, in the glass melt as well as their removal are described for example in “Glastechnische Fabrikationsfehler”, edited by H. Jebsen-Marwedel and R. Brückner, third edition, 1980, Springer Verlag, on pages 195 ff.
Chemical fining methods are most often used. Their principle of operation is that compounds which undergo decomposition and release gases, compounds which are volatile at elevated temperatures or compounds which release gases in an equilibrium reaction at higher temperatures are added to the melt.
Sodium sulfate, for example, which is used for fining soda-lime glass, belongs to a first group of these compounds used for chemical fining. In this case, the release of SO2 and O2 takes place in a temperature range of 1300° C. to 1450° C. with a maximum at 1380° C. This temperature range approximately corresponds to the fining range of such glasses.
Compounds, which are volatile at high temperatures due to their vapor pressure and thus act as a gas, such as halides, sodium chloride or different fluorides, belong to a second group. Thus for example a series of boron silicate glasses are fined with sodium chloride. The fining agents of this second group are designated “volatilization fining agents”.
A last group of fining agents comprises the so-called redox fining agents, such as arsenic oxide and antimony oxide. In practice, they are used most often. In the respective fining methods covalent ions, which can be present in at least two inter-related oxidation states in a temperature-dependent equilibrium with each other so that a gas, usually oxygen, is released at high temperatures, are used as redox fining agents.
For arsenic oxide as an example, the redox equilibrium that occurs in the melt can be represented by the following equation (I):As2O5As2O3+O2↑  (I),
The equilibrium constant K for equation (I) is given by equation (II):
                              K          ⁡                      (            T            )                          =                                                            aAs                2                            ⁢                                                O                  3                                ·                                  z                  ⁡                                      (                                          O                      2                                        )                                                                                                      aAs                2                            ⁢                              O                5                                              .                                    (        II        )            
In this equation aAs2O3 and aAs2O5 represent the respective activities of the arsenic trioxide and arsenic pentoxide and z (O2) represent the fugacity (of the oxygen). The equilibrium constant K strongly depends on the temperature and a defined oxygen fugacity z (O2) can be adjusted with the temperature and the activity of the oxidic arsenic compounds.
A disadvantage of many redox fining agents is that they are harmful to the environment; at least they are not environmentally acceptable. Particularly this applies to arsenic oxide. However this particular fining agent has a property which is useful: The release of the fining gas O2 has two maxima at about 1250° C. and at about 1600° C. These maxima correspond to the temperature range of melting, on the one hand, and the temperature range of fining, on the other hand. This sort of fining behavior is desirable, but arsenic trioxide is strongly toxic and undoubtedly classified as carcinogenic. Anionic arsenic is already present as arsenite and arsenate in high concentrations in the ground water of many countries. This is the reason that As2O3 is to be forbidden in several countries in the next years.
DE 1 596 860 discloses a method for the production of transparent glass ceramics having low thermal expansion. As a fining agent 0.3 to 0.5% by weight of As2O3 is used. Environmentally acceptable fining agents are not discussed.
U.S. Pat. No. 4,851,372 describes transparent glass ceramics which are fined with 0 to 1.5% by weight of As2O3 or Sb2O3. Also in this case, there is no discussion about environmentally acceptable fining agents.
EP 1 864 952 A1 describes aluminium silicate and lithium aluminium silicate glass ceramics containing amounts of SnO2 and/or CeO2 of 0.01 to 5.0 percent by mass each, based on the oxides. However, the cerium oxide affects the transmissions of the glass ceramics.
US 2007/0281849 A1 describes glass ceramics containing amounts of SnO2 and/or CeO2 of 0.01 to 5.0 percent by mass, based on the oxides. The presence of cerium oxide is a disadvantage, because it may result in an undesired coloration.
DE 199 39 771 A1 describes a fining method for glass melts during which a fining gas is produced in the glass melt by a fining agent. As fining agents redox compounds, in particular redox oxides such as SnO2, CeO2, Fe2O3, ZnO, TiO2, V2O5, MoO3, WO3, Bi2O5, PrO2, Sm2O3, Sb2O5, Eu2O3, TbO2 and/or Yb2O3, can be used. In addition, metal oxides, such as ZnO, As2O3, Sb2O3, Bi2O3 and/or SnO, are added, which release oxygen and change into the metallic state, during the fining operation. Many of these fining agents and their combinations, such as CeO2 and Fe2O3, change the transmission properties of the glass ceramic in a disadvantageous manner.